Added code to load file formats after construction

In order to do delayed loading the openFile function must return once it
knows if it can read the file or not, rather than calling processEvents.

Change-Id: I7f06a9161c545ac59a4cddf547854dc732442fe1
Updated and expanded the MainWindow documentation

Change-Id: Ieb7defcb89b5d7084a9960339e6c2773b56db00a
Add a slot for reader completion, return result from openFile

Added a slot to receive the signal when the background reader completes.
changed the signature of openFile to return a bool indicating whether
the requested file could be opened or not.

Change-Id: Ib7a16849d4ac701d2cd150ff7f25c9c9aa2ef46e
Merge topic 'asynchronous-formats' into master

1e36333f Add a slot for reader completion, return result from openFile
bd87f74b Updated and expanded the MainWindow documentation
0233e14d Added code to load file formats after construction